transmission on an 04 duramax

i just bought an o4 chevy duramax with a 112,000 miles on it ... my question is if i replace the tiny stock tires ( 245/75/16)  with a slightly larger tire (285/75/16) will this hurt the drive train at all ( i do understand that it lowers the gearing in the truck and throws of the speedometer some but is there ANYTHING that i should have to worry about? or should i just accept the fact that the truck looks fugly with those tiny tiny tires ( also want a 4" inch lift so clearance wont be a broblem.

thanks  

Your drivetrain can handle that, however in my opinion, I think that the larger tire size you cited may be a little bigger than what I'd put on, and what a normal tire shop would recommend. Also tires/wheels do not impact gearing in the mechanical sense, but rather the speedo reading because the number of revolutions your wheels/tires will make is going to be different. If you go from a 245 to 285, I think your speedo is going to be reading somewhere from 3-5 mph slower than normal.  

You could go with 285, but my father-in-law went from 245 to 285, and this last time around settled on 265s, and he said going from 285 to 265 noticeably improved his acceleration and fuel economy.

The Allison is a pretty stout transmission. Unless you've done any major performance modifications to the engine, you should be fine.

If you're going with a lift in addition to bigger tires, you may want to look at getting the truck re-geared if you're pretty sure you're going to keep it lifted.  

I wouldn't worry about it...most HD trucks tranny's can handle up to 35" (315) tires with out much strain really.

Diesels don't really need the re gearing like gas engines do due to the massive amounts of torque you have.

Now once you go modding your diesel well...even that allison will tear up after so much increase. I found that out the hard way.  
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2006 Dodge Ram 2500 Cummins 4X4 Sport (Heavily Modified 12.476 @ 108.96)
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1973 Dodge Rallye Challenger 340 (13.7 @ 102mph)